Every moving object has momentum. The more mass it has and the faster it goes, the more momentum it carries, and the harder it is to stop.

momentum = mass × velocity
p = m v

Momentum p in kilogram metres per second (kg m/s), mass m in kg, velocity v in m/s. AQA, Edexcel and OCR: recall this one (it is Higher tier at AQA).

1. A 70 kg runner moves at 6 m/s
2. p = 70 × 6
3. p = 420 kg m/s

Rearrange it the usual way: v = p ÷ m and m = p ÷ v.

Change the numbers into kg and m/s before you multiply.

1. A 46 g ball at 50 m/s: 46 g = 0.046 kg
2. p = 0.046 × 50 = 2.3 kg m/s
3. A 1200 kg car at 72 km/h: 72 ÷ 3.6 = 20 m/s
4. p = 1200 × 20 = 24 000 kg m/s

Forgetting to change grams into kilograms makes the answer 1000 times too big. Check: a ball should have a small momentum, a car a big one.

Momentum is a vector, like velocity and force. On a straight line, pick one direction as positive. Momentum the other way is negative.

1. Right is positive
2. A: 2 × 3 = +6 kg m/s
3. B: 1 × (−4) = −4 kg m/s
4. total = 6 + (−4) = +2 kg m/s, so 2 kg m/s to the right

Adding the two sizes (6 + 4 = 10) is the classic slip. Objects moving towards each other have momenta with opposite signs.